Rolling bearings and other geometrically simple rolling components are formed of SUJ2 or similar bearing steels providing long rolling contact fatigue life. Bearing steel, however, is poor in workability and unsuitable for rolling components having complicated geometries. In contrast, S53C and similar medium carbon steels have satisfactory workability and are suitable for rolling components having complicated geometries. Typically, medium carbon steel is worked into a complicated geometry and then has a rolling portion induction hardened for use. Furthermore, medium carbon steel contains an expensive alloy element in a small amount. It is thus inexpensive and also contributes to saved rare resources.
Rolling components having complicated geometries, however, often receive not only a simple rolling load at the rolling portion. In addition to rolling, there are also slippery and repeated tensile stress other than rolling superimposed. As such, the rolling portion is prone to cracking. This invites early propagation of cracking and can result in fatal damage. This is considered to be attributed to that medium carbon steel has a shorter rolling contact fatigue life than bearing steel.
Recently, as energy conservation and miniaturization are pursued, rolling components increasingly tend to be used under severer conditions than before. Bearing steel, poor in workability, has its limitation in providing long life with productivity and cost considered, and there exists an increasing demand for a rolling component produced from a source material provided by a steel material corresponding to conventional medium carbon steel with its inexpensive alloy components C, Si and Mn modified in content. More specifically, there exists a demand for the following items (1) and (2):
(1) As seen at the medium carbon steel level, the induction hardened portion has a rolling contact fatigue life improved to be comparable to that of bearing steel; and
(2) As seen at the medium carbon steel level, the induction hardened portion has increased surface cracking resistance.
Item (1) is effective in improving reliability against rolling fatigue and item (2) is effective in reducing surface cracking attributed to slippery.